Feed locus for semiparabolic reflector



Oct. 20, 1953 c. v-: ROBINSON 2,656,464

FEED LOCUS FOR SEMIPARABOLIC REFLECTOR Filed Nov. 5, 1945 INVE OR CHARLES V.- ROBIN N ATTORNEY semi pa-rabolic pill box.

Patented Oct. 20, 1953 FEED LOCUS" FOR SEMIPARABOLIC REFLECTOR Charles V. Robinson; Newton Center, Mass, as-

signor, by mesne' assignments, to the United States. of America. as represented by the Secrotary of the Navy Application November 5, 1945, Serial N 0. 626,844

11 Claims. (01.,250-3335) This invention relates to microwavescan antennas and in. particularto a method or feeding a semi-parabolic reflector interposed between and in contactwith two-parallel plates or wave guide members commonly knownas a pill-box, with: energy from a movable wave guide horn to produce a scan While-maintaining satisfactory definition in the output beam.

It-is the object of this invention to construct a semi-parabolic pill box reflector having a wave guide feed which is so-di-rected and oscillated or rotated along a locus in the region: of the reflector focus that an output fan beam of radiation will scan or tilt corresponding to motion of the read without excessivedegeneration of the directive pattern. The invention accomplishes this by directing the end of a wave guide feed from the focus at an angle toward the semiparabola' and causing the feedtube moved in a portion of a properly chosen circular are so that the" beam definition is preserved.

Instead of using-a reciprocating motion of the feed along the locus the rolled throatfprinciple of. construction may be applied so that a complete rotary motion may be used to feed the For a more detailed description of the rolled throatidea reference may be had to patent application of Roy Clarence Spencer entitled Rolled Throat Scanning Feed Horn, Serial Number 628,283, filed November 13, 1945.

Fig. l is a diagram illustrating the essential principle of this invention of properly choosing the path of travel of the feed in order to main-,

tain definition of the reflected beam from semi-parabolic pill box reflector; and

Fig. 2 is a perspective of a semi-parabolic pill box having a cylindrically rolled throat around and in which a wave-guide feed is continuously revolved to produce a scanning beam.- In Fig; '2 there is also shown an auxiliary parabolic cylindrical reflector 54 for changing direction and for providing better definition of the beam in the vertical plane.

Referring now to Fig.'1,"there1is illustrated a reflector 2 having a semi-parabolic shape with its focus at 4. The reflector '2 is to be understood as included between two parallel plates, each plate bounded by an edge of reflector 12 and by the aperture edge indicated as 6 and axial edged. This structurein microwave technology has come by usage to be commonly called a pill-box. At thefocus 4 is located a wave guidefeed horn It tiltedso that it directs energy toward some point [2 in the mid region of the semi-parabolic'reflector Zproducing a radia :tion beam M in a direction desired as the central axis of propagation of the directed beam, generally parallel to the parabolic axis. This 5 point 12 may be referred to as the mid-scan .point of illumination, ,or as lying in the midregion of the parabolic reflector.

Lateral motion of the feed k while it is kept :pointed. substantially at'ipoint l2 .will cause a tilting of the output beam, as illustrated. This invention teaches that in order to preserve best definition of the output beam there is. an optimum locus of travel of the feedhorn l0, which locus of travel is in the neighborhood of the focus :1 along an ,arcofa circle. 16 which passes through the focus and which also is tangent to .the parabola 2 at the point 12. The feed l0 travels over only a relativelysmallarc, for example between points [8 and 20, and may therefore actually follow a linear path 2| drawn tangent to the locus: circle at the focus without serious deteriorationof the-beam definition. Fig. 2 shows in perspective an embodiment of .the invention having an aperture region into .Which the feedhorn directs energy, the aperture beingoylindrically rolled into a throatvmember in order that the feed horn may employ a rotary motion within the throat or aperture region. This antenna-has a semi-parabolic pill box structure 30 composed of two parallel plates 32 and 34 with parabolic reflector 36 as its back wall. The aperture or throat region of the pill-box 1 structure 3llis rolled'oylindrically with the cyl- -inderelements parallel tothe aperture, or otherwise stated perpendicular to the parabolic axis. Dotted line il indicates the cylinder'axis. In this way the feed. locus becomes an annular opening between the forward portions of plates 32 and 34. Points 42 and 44 thus are. the approximate equivalent ;of points 18 and 20 re- :spectively otFig. .1 but onthe linear feed travel path. A-wave guide feed horn-46 is inserted in this annular opening or rolled throat 38'and has a bend 68 in it (or otherwise is cut on the bias) 45 so that, as in the case of :Fig. l,-it directs its .en-

'ergy substantially toward the properpolntlon the .semiparabolic back-wall 36 of the pill-box 30 which acts to reflect the energy so that it travels 'substantiallyparallel to the parabolic axis to the oo aperture 56 at the mouth of the pill-box. A 1 partial folder partialreversecylindrical. roll .52 in the parallel, plates directs the energy against an auxiliary parabolic cylindrical greflector 54. Reflector 54 serves to improve thedefinition in the vertical plane as here shown. When the feed guide 48 revolves around and around the cylinder axis M (by conventional means not shown) in the rolled throat 38 the output beam from reflector 54 is caused to tilt back and forth correspondingly. The feed locus within the cylindrical throat portion 38 is again in effect substantially a portion of a circle which passes through the focus of the parabola 35 and tangent to the parabola 36 at the point toward which the feed 46 is directed in the mid-scan position. Actually the feed path is the linear feed path rolled up rather than of the arc, which really only serves in this embodiment to define the position and orientation of the linear feed path.

To produce an efiicient horn of this type a given semi-parabolic reflector is illuminated by a wave guide and feed located at the focus and pointed at various angles to determine which yields the best gain. Horns of various flares are obvious equivalents to achieve the same objective. The mid-scan point 12 is ascertained for each such modification. By taking the circle which passes through this point tangent to the parabola, which also passes through the focus, the feed locus is ascertained. Such portion on either side of the focus may be used as is needed to produce the degree of tilt required, limited of course by the degree of definition also required. This portion of the pill-box throat structure is then rolled up cylindrically as above discussed to provide a particular pattern of tilt of propagation direction for the revolving feed horn.

What is claimed is:

1. In a microwave antenna for producing a scanning beam, a semi-parabolic reflector surface positioned at one end of a parallel plate, wave guide member; a movable wave guide feed horn for illuminating said semi-parabolic reflector surface in said wave guide member; and means for mounting and pointing said feed horn from the focus toward a point in the mid-region of said reflector and for moving said feed horn along a portion of an arc of the circle which is tangent to the reflector at said point and passes through the focus, whereby a scanning output radiation beam is produced at the aperture of said wave guide member.

2. In a microwave antenna for producing a scanning beam, a semi-parabolic reflector surface interposed at one end of a parallel plate wave guide member having an aperture at the other end, a movable wave guide feed horn for illumihating said semi-parabolic reflector surface of said wave guide member, and means for mounting and pointing said feed horn from the focus toward a predetermined point of said reflector and for causing said feed horn to repeatedly traverse a path in the region of the focus, which path is a relatively short portion of the straight line tangent, at the focus, to a circle which passes through the focus and which circle also is tangent to said reflector at said predetermined point, whereby as a result of such motion of the feed horn a scanning output beam is produced at the aperture of said wave guide member, said predetermined point being that point toward which the feed horn must point in order to obtain optimum gain.

3. In :a microwave scan antenna, a parallel plate, wave guide member having a semi-parabolic back wall reflector, and having its rectangular aperture region shaped and cylindrically rolled about an axis parallel to said wave guide member, aperture for providing an annular feed throat adjacent to the focus of the reflector, a revolvable wave guide feed horn fo projecting a beam of microwave energy therefrom through into said throat and against a point in the midregion of said reflector, and means for revolving said feed horn about said axis whereby a scanning output beam is produced from said aperture.

4. In a microwave scan antenna, a parallel plate, wave guide member having an aperture and a semi-parabolic back wall reflector, and having its aperture region shaped and cylindrically rolled about an axis parallel to the rectangular wave guide aperture for providing an annular feed throat adjacent to the focus of the reflector, a revolvable wave guide feed horn for projecting a beam of microwave energy therefrom angularly into said throat and toward a point in the midregion of said reflector, said point being that point toward which said beam of microwave energy must be directed by said feed horn in order to obtain optimum gain, means for revolving said feed horn about said aixs whereby a scanning output beam is produced from said aperture, and an auxiliary parabolic cylindrical reflector positioned adjacent to said aperture for increasing the beam definition in the plane perpendicular to the scanning plane.

5. A microwave scan antenna, comprising a parallel plate wave guide member having :a semiparabolic back wall reflector, a movable wave guide feed horn positioned in the region of the focus of said reflector and electromagnetically pointed substantially toward a point in the midregion of said reflector, said wave guide member having its aperture region shaped and rolled cylindrically to provide an annular feed throat which is in effect a rolled up portion of two parallel straight lines, tangent respectively at the focus to circles passing through the focus and tangent to said reflector at said point, and means for revolving said feed horn around within said annular throat, whereby a scanning output beam is produced.

6. A microwave antenna for producing a scanning beam of electromagnetic energy comprising a wave guide member having a parabolic reflecting wall at one end thereof, a hollow substantially cylindrical feed channel having a rectangular lateral aperture opening into said wave guide member at the open end thereof, a reverse cylindrical roll comprising an extension of said feed channel for directing said energy into an antenna reflector, an auxiliary cylindrical parabolic reflector positioned proximate said reverse roll for increasing the beam definition in a plane perpendicular to the scanning plane, and a directive wave guide feed horn opening into said feed channel and circularly rotatable therein, the directive feed horn being disposed angularly therein to direct energy toward said parabolic reflecting wall, whereby a scanning output beam is produced as rotation of the horn in said channel1 alters the direction of reflection from said wa l.

7. The antenna of claim 6 wherein the wave guide feed horn rotates circularly within said cylindrical feed channel in a locus including electromagnetic focal point of said parabolic reflec- 5 3. The antenna of claim 6 wherein the wave guide feed horn contains an angular bend to beam said energy toward the mid-region of said wall, whereby lateral displacement of beamed energy impinges on said wall at varying angle.

9. A microwave antenna for producing a scanning beam of electromagnetic energy comprising a wave guide member having a parabolic reflecting wall at one end thereof, a feed throat rolled into hollow cylindrical form arranged to receive a circularly rotating feed horn in one end thereof and having a rectangular aperture along one side communicating with an opposite end of said wave guide member, a flared horn longitudinally opening into said feed throat positioned adjacent said wave guide member, an auxiliary cylindrical parabolic reflector fed by said flared horn, and a continuously rotatable wave guide feed horn projecting a beam of energy angularly into said feed throat toward a. point in the mid-region of said reflecting wall, whereby a scanning output beam is produced by a change of the reflective angle at said wall as the feed horn is rotated in the cylindrical throat.

10. The antenna of claim 9 wherein the cylindrical rolled member is disposed perpendicularly to the principal axis of the parabola of said References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,419,024 Iams Apr. 15, 1947 2,429,801 Biskeborn Oct. 28, 1947 2,436,408 Tawney Feb. 24, 1948 2,442,951 Iams June 8, 1949 2,504,333 Iams Apr. 18, 1950 

